Layered double hydroxides (LDHs), which belong to a typical family of anionic layered materials, can be represented by the general formula [M2+1-xM3+x(OH)2]x+(An−)x/n.mH2O, where M2+ is a bivalent metal cation, M3+ is a trivalent metal cation, An− denotes an interlayer anions with negative charge n, m is the number of interlayer water molecules, and x is the molar ratio of the M3+ to the sum of the M3+ and M2+. The identity and ratio of the layer elements as well as the interlayer guest anions can be adjusted over a wide range in order to obtain materials with specific structures and properties. Because of their flexible composition and versatility, LDHs have been widely investigated for their potential applications in the fields of catalysis, adsorption, ion exchange and functional materials.
LDHs are traditionally synthesized by coprecipitation methods, hydrothermal methods, ion-exchange methods or calcination-rehydration methods. A large amount of sodium salt is produced as by-product during the preparation of LDHs by traditional methods. The sodium salt mother liquor is always discharged directly due to the high energy costs of evaporation, and thus leads to environmental pollution. In addition, the use of strong alkali in the synthesis process means that the product must be well washed with water (tens or even hundred times of the product's mass), which leads to significant waste of water and problems with treatment of the alkaline effluent. Thus it is necessary to develop an environmentally friendly technology for preparation of LDHs.
The coprecipitation method is the most popular method used to prepare LDHs. A mixed salt solution containing the metal ions which constitute the layers are coprecipitated with alkali in order to obtain the LDHs. Either the mixed salt solution or the alkali solution can contain the corresponding interlayer anionic group. In a related reference (Y. Zhao, F. Li, R. Zhang, D. G. Evans, X. Duan, Preparation of layered double-hydroxide nanomaterials with a uniform crystallite size using a new method involving separate nucleation and aging steps, Chem. Mater., 2002, 14:4286-4291), a method for synthesis of LDHs has been reported, in which LDHs are prepared by coprecipitation of a mixture of soluble salts of bivalent and trivalent metal ions with Na2CO3 and NaOH. However, in this method, large quantity of water is required to wash the product after the reaction, due to the large amount of sodium salts produced in the reaction and the strongly alkaline solution, and a significant waste of water is thus caused.
The hydrothermal method for preparation of LDHs is a method in which insoluble oxides or hydroxides containing the metal ions to be incorporated in the layers are treated with water at a high temperature under a high pressure. In this method, Na2CO3 or NaHCO3 is generally used as main starting materials, and the sodium salt formed as a co-product needs be removed by washing which causes a lot of water waste. In a related reference (Zhi Ping Xu, Guo Qing (Max) Lu, Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from Mixed MgO and Al2O3: LDH Formation Mechanism, Chem. Mater. 2005, 17:1055-1062), a process for preparing MgAl—CO32−-LDHs has been reported, in which Na2CO3 or NaHCO3 is added into a mixture of MgO and Al2O3 at 110° C., and the product contains a lot of Na+ which needs to be washed with large quantity of water.
The ion-exchange method is used when M2+ and M3+ are not stable in alkaline medium or no suitable soluble salt of the anion An− can be found. An LDHs precursor is first synthesized and the ion-exchange reaction is then carried out in the presence of the required interlayer anions under appropriate conditions in order to prepare the target LDHs. In this method, the washing process cannot be omitted due to the formation of salt by-products in the production of the precursor. WO 2007/051377A1 discloses a method for preparing LDHs with interlayer organic anions containing double bonds, in which an LDHs precursor containing interlayer NO3− anions is first synthesized by the coprecipitation method, and after washing and filtration, LDHs containing organic anions containing double bonds is obtained by the ion-exchange process.
In the calcination-rehydration method, complex metal oxides (LDO) are obtained by calcination of an LDHs precursor, and the LDO is added into a solution containing the desired anions to restore or partly restore the ordered layered structure of LDHs. Generally, it is possible to restore the ordered layered structure when the calcination temperature is below 500° C. When the calcination temperature exceeds 600° C., a spinel phase is formed from which the layer structure of the LDHs cannot be restored. An LDHs precursor must also be synthesized for use in the calcination-rehydration method and therefore the washing process cannot be omitted due to the formation of salt by-products. In a related reference (Wei Jiang, LanPing Nong, WenLing Lai, ZeYu Chen, Intercalation and assembly of organic acid radical-pillared layered double hydroxides by calcination-rehydration, Chemical Research and Application, 2004, 16(6): 828-830), LDHs with anions of myristic acid or stearic acid as the interlayer anion were prepared by synthesizing MgAl-LDHs and ZnAl-LDHs precursors with the coprecipitation method, calcinating the precursors to obtain LDO, and then putting LDO in myristic acid or stearic acid solution. In the process of preparing the precursors, a large amount of by-product is formed and large quantity of water is required for washing.